In medical magnetic resonance investigations, it is often first decided on the basis of a general scan which area of a patient is to be imaged. This involves a user graphically determining an area for the next magnetic resonance measurement on the basis of a magnetic resonance image for example. On account of inhomogeneities in the base magnetic field and gradient field of the magnetic resonance device, distortions occur in magnetic resonance images. The distortions are manifested as image defects, which make graphic positioning of the next magnetic resonance measurement more difficult.
Distortions caused by inhomogeneities of the magnetic fields can be reduced or eliminated by suitable methods of correction. However, the positioning of a slice of the subsequent magnetic resonance measurement is not possible on such distortion-corrected images. The slice planned on the basis of the distortion-corrected images generally deviates from the actually measured slice because of the magnetic field inhomogeneities, for which reason the desired slice would not be measured in this case.
In specific situations, this can constitute a risk for the patient, for example in the case of a biopsy. If a biopsy needle is positioned on the basis of a magnetic resonance image on which the desired slice is not depicted, this may result in the removal of an incorrect tissue sample or in injury to the patient. For this reason, so far distortion-corrected magnetic resonance images have not been used for slice positioning and positioning has been carried out on the basis of the uncorrected images.
U.S. reference 2004/0160221 A1 describes a method for exciting planar slices in a magnetic resonance device by taking into consideration non-linear gradient fields. In the process, at least one magnetic resonance overview image is initially created, which is followed by a correction of the overview image on the basis of stored data with respect to non-linear deviations of the gradient fields.
For a planar slice, selected by a user in the corrected overview image, a deviation that occurs in the selected slice is determined on the basis of the stored data. A magnetic resonance sequence with high-frequency excitation pulse, as well as gradient pulses, is computed in such a way that with a simultaneous irradiation of these pulses, the slice to be recorded experiences a curvature during the recording, which is inverse to the non-linear deviations of the overview image. A planar slice is then recorded corresponding to the computed magnetic resonance (MR) sequence.